The rumblings from last week’s Alaskan earthquake have an impact on the numbers in the week of 17-23 March 2016 — over 30 of at least magnitude 2 (≥M2.0) appear on the United States Geological Survey’s real time earthquake map for the week.
The map, which includes (broadly speaking) tremors of all magnitudes in the US and its territories and those of ≥M4.0 elsewhere, recorded a total of just over 2,000 seismic events this week, of which three, including one of the Alaskan aftershocks, were ≥M6.0; 20 ≥M5.0; and 118 ≥M4.0.
Once again the distribution of these earthquakes failed to throw up any surprises. Most were concentrated around the margins of the Earth’s tectonic plates and those which appear as outliers are associated either with known zones on tectonic extension or with the edges of large areas deformation resulting from collision.
The Week’s Largest Earthquake: M6.4, Kamchatka
The Pacific is surrounded by highly active volcanos and seismic zones — the so-called Pacific Ring of Fire — where the tectonic plates which make up the ocean floor are gradually being consumed as they descend beneath the surrounding continents. It’s no surprise, then, to find that this week’s largest earthquake, like last week’s, is not just in the Pacific but in the north of the ocean.
The tectonic setting of this week’s M6.4, off the Kamchatka peninsula, is similar to last week’s in Alaska, in that the Pacific plate is subjecting beneath the North American plate. Subduction here has a different direction (east-west rather than south-north) and is along a different section of the margin (the Kuril-Kamchatka trench rather than the Aleutian Trench) but in other respects the causes are similar.
The earthquake took place at the northern end of the subduction zone and its depth (31km) and location (its epicentre was roughly the same distance from the trench on the over-riding plate) suggest that it’s an example of a straightforward, medium-sized subduction earthquake caused by movement at or near the plate interface.
M6.0 Quake: Barbuda
Last week I discussed the Atlantic and its two short subduction zones. By comparison with the Pacific, the Atlantic is relatively quiescent — but this week an earthquake of M6.0 just off the island of Barbuda illustrates that ‘relatively quiescent’ doesn’t mean no significant seismic events can ever occur.
The earthquake occurred at the eastern edge of the Caribbean plate, where the North American plate (probably — the boundary between it and the South American plate is undefined) subducts beneath it.
Yeats notes that seismologists have estimated a maximum earthquake size here of around M7.5, which makes this week’s M6.0 noteworthy but not exceptionally so. Its location relative to the margin and its depth are almost exactly the same as the Kamchatka event above, implying a similar, plate-interface, origin.
US Earthquakes: Nevada
Oklahoma, the usual suspect, is quiet this week, leaving Nevada with the largest tremors in the contiguous states — two of at least M4.0. Two minor clusters appear on the USGS map, either side of Hawthorne, Nevada.
Rumblings of this sort have been going on in the area for years, with various upsurges. The Rockies, and its so-called basin and range topography, is a classic area of extensional tectonics where minor earthquakes, and large swarms of them, are by no means uncommon.
Last Thoughts: Number Crunching
This week an error message in my search of the map (This search includes more earthquakes than your device can display) brought me to do something I should have done ages ago — confront the numbers.
My standard disclaimer is that in the opening paragraph above — that the map, which includes (broadly speaking) tremors of all magnitudes in the US and its territories and those of ≥M4.0 elsewhere. This is a useful (and reasonable) working definition but in fact the USGS has a whole pageful of caveats, noting that the map: “should not be considered to be [a] complete list of all events in the US and adjacent areas and especially should not be considered to be complete lists of all events M4.5+ in the world”.
I’m guilty of simplification: I confess (who isn’t?). But earthquakes are complicated things.